Calcium sulfate is known to exist in several different chemical forms generally differing in gross chemical composition only by the amounts of hydrogen and oxygen expressed as water (i.e. in the same 2 to 1 ratio as in water) relative to the amount of calcium sulfate expressed as the simple salt, CaSO.sub.4. In this way, the art recognizes at least; a hydrate (CaSO.sub.4.2H.sub.2 O); a hemihydrate (CaSO.sub.4.1/2H.sub.2 O); and, an anhydrite (CaSO.sub.4). Additionally, the anhydrite and the hemi-hydrate exist in at least two different forms, based on different degrees of water solubility. In addition to the several different chemical forms that have been proposed for calcium sulfate, it is known to be polymorphous as well, having at least two distinct crystalline forms; rhombic and orthorhombic or monoclinic. Other crystalline variations including acicular, needle-like and whisker crystals have also been recognized. Reference to such variations in physical form, as ribbon or tape-like crystals, columnar and rod-like, twinned and swallowtailed, curved and prismatic, are frequently encountered in discussions of calcium sulfate or gypsum as the hydrated form is commonly known. Thus, it appears that authors disagree as to the exact chemical composition and the physical form of the various transformation products of gypsum. The numerous attempts to define the various forms of the product and its transformation mechanism have not removed the uncertainty as to an understanding of the composition, structure, properties, and behavior of calcium sulfate, nor provided a generally accepted correlation between them.
There, nevertheless, have been numerous attempts to explain the behavior of calcium sulfate either by transformation of physical forms or by the addition or loss of water of hydration; see for example, The Chemistry of Cement and Concrete by Lea and Desch (Longmans, Green & Co., New York, 1935), pages 17 to 20; Journal of The Society of The Chemical Industry No. 13, Vol. XXVI (July 15, 1907), pages 727-738; U.S. Pat. No. 3,594,123; U.S. Pat. No. 1,901,051. Elsewhere, such as J. Appl. Chem. 1968, pages 307 to 312, differences in various forms of calcium sulfate formed in autoclaving are attributed to differences in such things as purity, particle size and thermal history of the gypsum starting materials.
The commonly used crystalline form of calcium sulfate known as plaster has long been an important product of the rehydration of dehydrated gypsum stored as a powdered hemi-hydrate or anhydrite which has the ability to set when rehydrated by simply mixing with water. This property has been widely exploited by a variety of products, the most common of which is known as Plaster of Paris. Other gypsum products are known commercially as Keenes plaster, Gips and a multitude of proprietary names. These products, which generally are stated to be the hemi-hydrate or anhydrite form, are generally manufactured by dehydrating gypsum. The Plaster of Paris or similar plaster product can be made by several different processes. One commonly used process involves autoclaving ground, natural gypsum in an atmosphere of steam. All of the commercial methods for making Plaster of Paris generally seek to obtain a powdered product of uniform particle size with individual particles of minimum surface area; this is mostly done by making the particles as cubic as possible so that the minimum amount of water is necessary to wet the surface for rehydrating. In preparing these products, particularly in methods employing autoclaving, there are frequent references to the formation of undesirable acicular or needle-like crystals. Early references to needle-like crystals formed in producing Plaster of Paris can be found for example in U.S. Pat. Nos. 757,649 and 782,321. Manufacturers of Plaster of Paris products encountering the problem of acicular crystal formation sought to eliminate them either in avoiding the conditions of their formation or in breaking them up by pulverizing.
Still others have experienced the formation of needle-like crystals in autoclaving gypsum to produce plaster products where the presence of such fibrous material is generally regarded as undesirable. Thus, U.S. Pat. No. 3,410,655 is specifically directed to avoiding the formation of needle-like crystals. Only within about the last ten years has it been recognized that calcium sulfate whisker crystals could be advantageously employed as reinforcement fiber and that conditions under which acicular crystals form might be controlled to give high yield conversion of gypsum to whisker fibers having length to diameter ratio of at least 6 to 1 and as high as 100 to 1 or higher. Whisker fibers of this type have been produced for example as described by Eberl, et al in U.S. Pat. Nos. 3,822,340 and 3,961,105.
While these patents recognize a variety of uses for such fibrous calcium sulfate product, particularly as a substitute for asbestos fibers, none of the products made in accordance with these methods, has actually achieved commercial use. The lack of commercial acceptance is believed to be largely due to the difficulty in dispersing and effectively employing the fibrous product obtained as a tangled mass of fibers that cannot be readily separated. When reinforcement is attempted with such fibers they remain in the form of tangled lumps or balls and offer little practical benefit.
Accordingly, a high strength calcium sulfate fiber that is readily dispersible in liquid media is highly desirable and it is an object of this invention to provide an improved method for the production of calcium sulfate whisker crystals which can be readily dispersed for use as reinforcing fiber. A further object of this invention is to provide a method by which gypsum is converted into stable whisker crystals with average fiber length of 100 to 300 microns having strength and aspect ratio conducive to greater usefulness as reinforcement. It is a further object of this invention to provide a method wherein substantially all of the gypsum is converted to whisker crystals 50 microns long or longer and aspect ratio of 10 to 1 or greater which crystals are obtained as substantially discrete fibers easily separated and capable of being uniformly dispersed in liquid media using conventional techniques. These and other objects of the invention will be more fully understood from the description and examples which follow.